This invention relates to a solid electrolytic capacitor using an organic semiconductor as an electrolyte. More particularly, the invention relates to a solid electrolytic capacitor in which a polymer of a heterocyclic compound such as pyrrole, furan or thiophene is used as a solid electrolyte, and to a method of manufacturing this capacitor.
Conventionally, solid electrolytic capacitors use manganese dioxide (MnO.sub.2) as a solid electrolyte or an organic semiconductor such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) salt as the solid electrolyte.
A solid electrolytic capacitor in which manganese dioxide (MnO.sub.2) is used as the solid electrolyte has a capacitor element obtained by forming a thin dielectric oxidation layer such as aluminum oxide (Al.sub.2 O.sub.3) on the surface of a metal such as aluminum on which the dielectric oxidation layer is capable of being formed, forming a manganese dioxide layer (MnO.sub.2) on the thin dielectric oxidation layer, forming a graphite layer on the manganese dioxide layer, forming a silver paste layer on the graphite layer, using the metal as one electrode and using the silver paste layer as the other electrode.
A solid electrolytic capacitor in which an organic semiconductor such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) salt is used as the solid electrolyte has a capacitor element obtained by forming a thin dielectric oxidation layer such as aluminum oxide (Al.sub.2 O.sub.3) on the surface of a metal foil such as aluminum, overlapping an electrolyte paper and the metal foil and winding them into a coil, and impregnating the electrolyte paper with an organic semiconductor such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) salt or the like.
However, the solid electrolytic capacitor using manganese dioxide (MnO.sub.2) as the solid electrolyte is disadvantageous in that the process for forming the manganese dioxide (MnO.sub.2) layer on the thin film oxide of a metal is extremely complicated and costly. Another problem is that when the manganese dioxide (MnO.sub.2) layer is formed by the thermal decomposition of manganese nitrate, the oxide thin film is damaged. Though this can be repaired by reformation, the restorability of the oxide film is poor owing to the manganese dioxide (MnO.sub.2) layer. Moreover, since the conductivity of the manganese dioxide (MnO.sub.2) is low, the specific resistance or ESR of the solid electrolytic capacitor is large and the leakage current is high.
The solid electrolytic capacitor using an organic semiconductor such as 7,7,8,8-tetracyanoquino-dimethane (TCNQ) salt as the solid electrolyte is disadvantageous in that when the TCNQ salt is held in the heated and molten state, an insulating reaction takes place in a very short period of time (about 10 sec) and, when the salt cools and hardens, it is converted from a semiconductor to an insulator. Other problems are that the manufacturing process is difficult to manage and does not readily lend itself to mass production. In addition, the TCNQ salt is high in price and raises costs. A further drawback is that the specific resistance is high, though not to the extent of the solid electrolytic capacitor using manganese dioxide (MnO.sub.2) as the solid electrolyte.
Array-type electrolytic capacitors are also available. Conventional electrolytic capacitors of this type include thin-film array-type electrolytic capacitors, thick-film array-type electrolytic capacitors and sintered-type array chips. However, all of these capacitors have certain drawbacks, which will now be described.
The thin-film array-type electrolytic capacitors have a low rated capacitance in the range 100 pF-10,000 pF.
Though the thick-film array-type electrolytic capacitor has a higher capacitance of several microfarads, it is very difficult to manufacture owing to the fact that the desired anode circuit pattern is formed on an insulator substrate by photolithography or a masking process. At the same time, process control for forming a manganese dioxide thin film on the anode circuit pattern is very complicated and difficult to perform. If manganese dioxide is used as the electrolyte, moreover, the capacitor exhibits a higher specific resistance or ESR.
The sintered-type array chip generally has a low capacitance. Increasing the capacitance necessitates an array chip of larger dimensions.